Although ultrasonic waves have been used in medicine for diagnosis, therapy, and in a new form of microscopy, there has been virtually no work in controlling the forces that acoustic waves can exert on biological materials. Moreover, the limits of our ability to characterize biological media from scattered acoustic waves has not been reached. Our fundamental objective is the study of the fundamentals of such forces and scattered signals and their application in the characterization, deformation and manipulation of biological materials. Our initial work has been with red blood cells (rbc's). This choice should be viewed as a case study, as the techniques are readily generalizable to other cells and materials. We have shown that we can characterize rbc's by their elastic property. Furthermore, we have demonstrated that different materials (biological and otherwise) in a sound field can be spatially separated by their differing elastic properties. And, finally, we have demonstrated that at sufficiently high frequencies (into the megahertz range), individual cells can be manipulated by a sound field. In the proposed work we shall use acoustic waves to 1) characterize individual cells by their elasticity and density, 2) deform single cells and thereby deduce information on the cell membrane and perhaps internal structure, and 3) mainpulate intracellular components (such as the nucleus) relative to the cell wall. The characterization work will be performed by scattering pulses of high frequency ultrasound (25-100 MHz) from cells in solution as they fall past an acoustic focal region. By scattering from single cells we hope not only to determine distributions of cell properties but also to provide an opportunity for sorting out cells of different mechanical properties. Cell deformation and intracellular manipulatin can be performed with high frequency acoustic standing waves. Here an individual cell would be afixed to a specially designed stage and viewed with an optical microscope either as the deformation or manipulation takes place. Most of the proposed work will be carried out in a frequency range not ordinarily used for biomedical applications - i.e. above the range of diagnostic and therapeutic ultrasound and below the range for ultrasonic microscopy.